Doorbell systems only require a very low voltage to operate. The button on the doorbell completes a circuit that runs from a transformer, to the button, to the actual bell, and the transformer itself is fed from a standard 110-volt circuit in your home, just like a light or power outlet circuit. Different systems requires different voltages, generally either 12 or 16 volts. If you need to know what the voltage is in order to replace wires or entire doorbell components, you can look on the documentation or use a voltage meter.
Look on the package or instruction manual that came with your doorbell system. Those will specify whether the system uses 12 or 16 volts.
Look on the underside of the transformer itself if you don’t have the package. Transformers come labeled with the input they can handle, and the voltage they output. If the label is gone or inaccessible, go to the next step.
Touch the black wire of your voltage meter to the black wire coming out the transformer, and the white or red wire to the white wire coming out of the transformer. It will show a number close to either 12 or 16. If it shows a number much higher, like 110 or so, then you’re touching the input wires, not the output wires, so just try the wires coming out the other side of the transformer.
Touching a live circuit with a voltage meter is hazardous. Be sure to follow all the warnings and instructions that came with your voltage meter to avoid injury or death.